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EQUINE VETERINARY EDUCATION
Equine vet. Educ. (2020) 32 (11) 572-574 doi: 10.1111/eve.13140
Clinical Commentary
Metacarpal bone exostosis and associated suspensory ligament desmitis and adhesions: A review
C. J. Zubrod* and J. W. Kristek Oakridge Equine Hospital, Edmond, Oklahoma, USA
*Corresponding author email:
Zubrod@oakridgevet.com Keywords: horse; suspensory; exostosis; adhesion
Introduction
Exostosis of the second or the fourth metacarpal bones (splint bones) is a common condition affecting horses and is the subject of a case report by Owen et al. (2020) in this issue of Equine Veterinary Education. Causes of exostosis range from asymmetric or excessive loading, external trauma and fractures of the splint bones. Not all causes of exostosis warrant surgical therapy or advanced imaging. Many acute cases resolve with systemic anti-inflammatories, rest and topical therapy. It is important to determine whether the lesion is in the acute or chronic phase of inflammation to guide the diagnostic evaluation, treatment and prognosis. When cases do not respond to initial therapy or if the issue is recurring, further evaluation is required to identify occult splint bone fractures, exostosis with impingement on the suspensory ligament (SL) and/or adhesions between the exostosis and SL. While splint bone exostosis by itself can be fairly benign, these secondary sequelae can cause chronic pain for the case, be frustrating to definitively diagnose and may warrant surgical therapy for resolution of the lameness.
Diagnostics
The first step in accurate diagnosis is a thorough physical examination and lameness evaluation. Most commonly the medial splint bone is affected due to axial loading from the carpus and interference injuries with the contralateral limb. Watching the horse track in a straight line can provide insights into the cause of the exostosis as well as providing a baseline for comparison after treatment. Horses with a base narrow track are more likely to have interference injuries and are at risk for chronic medial splint bone injuries. A thorough distal limb examination is warranted to localise the lameness to a region and assess sensitivity to digital palpation. Distal limb flexion and consistent firm pressure applied over the suspensory ligament adjacent to the exostosis may increase the degree of lameness. Response to this manipulation may suggest adhesion between the splint bone and suspensory ligament or desmitis secondary to impingement from the exostosis.
Diagnostic regional anaesthesia is recommended to
localise the pain to the region of the exostosis. Systematic evaluation starting at the distal limb and working proximally is preferred. After ruling out causes of lameness distal to the exostosis, the area can be blocked in two fashions: local infiltration and regional nerve blocks. Local infiltration can be achieved by injecting an anaesthetic solution into the space
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between the exostosis and suspensory ligament and allowing diffusion to occur. This technique can interfere with ultrasound evaluation of the region and often it is recommended to wait 24–48 h post infiltration to obtain diagnostic images. Additionally, useofalargevolumeofanaesthetic solution will allow for diffusion of the anaesthetic to local nerves, which may complicate the diagnosis. The most definitive diagnostic nerve block is anaesthesia of the medial or lateral palmar metacarpal nerve on the affected side to block the ipsilateral splint bone, proximal suspensory and interosseous ligament. This can be performed by guiding a 22 gauge 1 inch (2.5 cm) needle between the suspensory ligament and the splint bone, just distal to the carpometacarpal joint in a nonweight-bearing position. Sterile technique is recommended due to the close proximity to the carpometacarpal joint and the carpal sheath. Desensitisation can be confirmed by lack of skin sensation distal to the site of injection and reduction of pain on deep palpation of the suspensory ligament and exostosis. Complete resolution of the lameness may not always occur.
Imaging
Numerous studies have been published on techniques for imaging the suspensory ligament in the horse including weight-bearing and nonweight-bearing techniques. The anatomy of the metacarpal region of the horse makes adequate visualisation of the abaxial margins of the suspensory difficult, if not impossible, with ultrasonography and has been well described (Werpy and Denoix 2012; Werpy et al. 2013). The thickness of palmar structures such as the superficial digital flexor tendon (thicker medially) and the deep digital flexor tendon (DDFT), inferior check ligament and accessory head of the DDFT (thicker laterally) affects the palmar contact surface for ultrasound imaging, meaning the axial margins of the SL are not easily imaged due to the contact surface being irregular. Secondly, the medial lobe is wider than the lateral lobe of the SL and in conjunction with the thickness of the palmar soft tissues, is more readily imaged. The use of a standoff can be used to improve the palmar contact surface but does not completely ameliorate the discrepancy. It is also important to recognise the effect of edge
artefact and through transmission on visualisation of the suspensory ligament beneath the palmar metacarpal arteries and veins. The anatomy of these vessels makes adequate visualisation of the abaxial margins of the SL challenging
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